1. Field of the Invention
This invention relates generally to internal antenna for portable communication system. More particularly, this invention relates to new configuration and method for designing and manufacturing a multi-resonance horizontal-U shaped antenna to broaden the bandwidth of an internal antenna for portable communication system.
2. Description of the Prior Art
Conventional techniques for employing conducting plates or strips configured in different shapes as antennas for transmitting and receiving electromagnetic waves of different frequencies are limited by a major difficulty that the bandwidth provided by the antennas for such transmissions are not sufficiently broad. The demand for broader bandwidth is ever increased for portable communication systems as more communication transmissions are rapidly increased and greater crowd of users are causing `traffic jams` in different frequency-bands.
Most of the traditional antennas employed by movable communication systems are extendible types including monopole antenna, whip antenna, sleeve antenna or normal mode helical antenna. It is well known fact in the art that the performance characteristics of these antennas are functions of the height of the antenna extended out of the rectangular metal box used to contain the communication system, e.g., a wireless telephone. Due the design considerations of simplicity, convenience of use, serviceability, reliability, and price, the external extendible antennas are increasingly being replaced by internal antennas. The internal antennas not only provides a design configuration to overcome the difficulties and limitations of the external antennas mentioned above, it further enhances the portability of the communication systems since the antennas can be conveniently placed in a pocket.
Among several types of internal antennas, the micro-strip antenna is most frequently being employed because of its low cost, small volume, light weight and easy to form on a flat surface. The microstip antennas however are limited by the narrow bandwidths. In order to resolve this difficulty, many other types of internal antenna are disclosed including the linear inverted-F antenna, planar inverted-F antenna. The linear inverted-F antenna was disclosed in early 1960s while the planar inverted-F antenna is to replace the linear radiation elements with planar elements. An inverted F-antenna can be considered as a quarter-wave microstrip antenna with air dielectric, matched by the RF-input stage by the position of the feed probe. Such feeds tend to restrict the antenna bandwidth defined by standing wave ratio, i.e., VSWR.ltoreq.2. Since the bandwidth as defined by the acceptable radiation patterns is usually much larger than the VSWR bandwidth, additional matching network elements can be added to expand the VSWR bandwidth such that it can approximate the radiation bandwidth. However, this expansion is at the cost of decreasing the radiation efficiency of the antenna caused by non-ideal components of the matching network
Rasinger et al. disclose a radiation coupled dual-L antenna which includes two narrow plates in the form of letter L on top of metallic shielding case. The narrow plates are arranged in parallel separated by a narrow slot. These two narrow plates are formed with equal length. This radiation-coupled dual-L antenna is intended to enhance the bandwidth of the antenna without requiring added matching network components thus enhanced bandwidth can be achieved without increasing the occupied volumes or consideration of matching network components. This enhanced bandwidth dual-L antenna has recently been published by Rasinger et al. in 1990 in IEEE Journal as attached herein. For ease of reference, please refer to FIG. 1A to 1E for different configurations of the prior art antennas as discussed above.
Rasinger et al. also applies a three-dimensional numerical analytical model i.e., an antenna wire-grid model to obtain calculated data of current distributions, 3-D radiation patterns, and input impedance at a fixed frequency. The analytical model enables an antenna designer to overcome the difficulties that experimental investigation of antenna performance is very time consuming as it depends on geometrical parameters and solder-intensive tinkering. Additionally, the measurement of radiation patterns in an anechoic chamber is very complex, expensive, and time consuming. The three dimensional wire-grid models, however, does not provide sufficient accuracy in defining the current distribution for a more complex antenna configuration. Furthermore, extra long computer execution time is required for performing such analyses when greater of number of grids is used. The technique of the 3-D wire-grid model is often not adequate to satisfy the need for modern antenna design.
Even that the dual-L antenna as disclosed by Rasinger is able to increase the bandwidth of the antenna without requiring the use of matching network components. However, since this dual-L antenna with the sum of the lengths of long and short arms equal to quarter wavelength, i.e., L+H=.lambda./4, it only has one resonant mode. The bandwidth enhancement is still very limited compared to the rapidly increased demand to accommodate a great number of users in the portable tele-communication market. Additionally, under the condition when the height of the antenna is less than quarter wavelength, i.e., H&lt;.lambda./4, the potential improvements on matching impedance for performance improvement is very limited by changing the position of the feed probe.
Therefore, a need still exist in the art of design and manufacture of antenna for portable communication system to provide a new antenna configuration and design technique for broadening the bandwidth and for improving the impedance match characteristics of an antenna such that limitations and difficulties as now faced by the art of antenna design for portable communication devices can be resolved and more effective and higher performance applications of the portable communication systems can be achieved.